Magnetic media are used for storage and retrieval of data, and come in many forms, such as magnetic tape and magnetic disks. Magnetic tape remains an economical medium for storing large amounts of data. A write/read head assembly, which includes one or more write/read transducer heads, reads data from and writes data to the magnetic medium. The data stored on the medium are usually organized into “data tracks,” and the transducer heads write data to and read data from the data tracks.
Data tracks on magnetic tape are generally parallel to each other, and often are oriented longitudinally on the tape. The head assembly is usually oriented transverse to the path of the magnetic tape so that the transducer heads move laterally across the width of the tape to access the tracks.
Data tracks on magnetic disks are typically arranged circumferentially. The head assembly is usually oriented so that the transducer heads move laterally across the tracks.
For efficient reading and writing, a transducer head must be accurately positioned to read from or write to a particular data track. A servo control system typically is provided to control the positioning of the head relative to the data tracks.
The medium often includes specialized tracks, called “servo tracks,” to serve as references or landmarks for the servo control system. The servo tracks are sensed by a servo read head, which may be narrower than the servo track. Once a particular servo track is located by the servo read head, a data track can be located on the magnetic medium according to the data track's displacement from the servo track.
Servo tracks can assume many forms. Conventional servo tracks store magnetic flux transitions of varying phases or frequencies. Time-based servo marks, by contrast, apply a different approach. Time-based servo marks are arranged in a series of patterns. A simple form of a pattern consists of a pair of marks, but patterns may include more than two marks. In many cases, the marks are not coincident with each other, nor are they parallel. Rather, the marks are angularly offset from one another, in a pattern such as a diamond or zigzag.